This invention relates to a method for detecting the state of humidity on a road, on which a motor vehicle is travelling under conditions of obscurity such that the lighting system of the vehicle is in use, the lighting system including, in particular, at least one driving light or headlight. The invention equally relates to apparatus carried on the vehicle for detecting the said state of humidity.
In the development of automotive lighting systems, a major modification is concerned with lighting in bad weather, and in particular the provision of a xe2x80x9cbad weather headlightxe2x80x9d. This is a dipped beam headlight which is arranged to give several levels of light distribution in accordance with the state of humidity or dampness on the road in front of the vehicle. It is also arranged to reduce the emission of light directed towards vehicles travelling in the opposite direction, and an increase in light intensity on one or both verges of the road.
U.S. Pat. No. 6,049,171 (U.S. Pat. No. 6,049,171) provides an onboard optical sensor in the vehicle, and a means for forming on this sensor an image of a zone of the road situated in front of the vehicle. If at least one vehicle is detected in this zone, the light intensity of the headlight is reduced. Humidity sensors are also provided to prevent excessive reflection of light emission from the headlights being reflected towards the driver. However, humidity sensors will detect rain drops on the windshield, but they do not permit the state of humidity of the road and/or of the atmosphere to be analysed.
In addition, it is known from French published patent specification FR 2 721 400A to provide a method and apparatus for detecting fog or smoke, for a motor vehicle, which comprises a camera mounted in the vehicle together with means for analysing the contrast of the points in the image. However, the state of humidity of the road and/or the atmosphere is not detected by this apparatus.
A main object of the invention is to provide a method for evaluating with accuracy the state of humidity of a road, with a view in particular to choosing the level of adjustment of a bad weather headlight. In such a headlight, the various levels of light distribution which are provided as a function of the road correspond to various states, which may for example be defined as follows:
a slightly damp road,
a very muddy road,
an intermediate state between the two foregoing states, and
rain.
In no way is it conceivable to envisage control of the headlight by simple visual appreciation on the part of the driver. The driver""s perception of the environment is too subjective, and it is imperative to arrange that the system will operate automatically on the basis of a method and apparatus which are adapted accordingly.
An object of the invention is therefore to provide such a method and apparatus which will give a reliable evaluation of the state of humidity on the road, and/or that of the atmosphere, while being relatively simple and inexpensive.
According to the invention in a first aspect, a method of detecting the state of humidity on a road on which a motor vehicle is driving under conditions of obscurity necessitating the use of the vehicle lighting system, there being an optical sensor fitted in the vehicle, the method including the steps of forming on the optical sensor an image of a zone of the road in front of the vehicle, is characterised in that the said zone of the road consists of at least one segment which is oriented in the direction of travel of the vehicle, whereby the image is a characteristic profile, in the form of a line or curve, representing the distribution of light on the road; the said image is compared with a reference in order to determine the state of humidity; and the said reference is the characteristic said curve, for the light distribution in at least one equivalent segment of dry road.
It is of course possible to rotate the optical sensor through 90xc2x0 and to associate with it a processing of the signal, for example by a digital signal processor, in order to achieve the same level of information.
Experience has shown that with the solution provided by the invention, the profile of the characteristic curve of the light distribution for a dry road (the reference characteristic) with that for a slightly damp or slightly humid road, intersect at a crossover point which is situated at a certain distance in front of the vehicle. The light distribution on the damp road is stronger than on the dry road closer to the vehicle than this crossover point, but becomes weaker than on a dry road at distances beyond the crossover point.
Preferably, the length of the road segment under surveillance is sufficient to cause a crossover point to be defined by the said characteristic profiles corresponding to a damp road and a dry road. This length may be about 15 meters from the headlight.
Preferably, the or each image of the zone under surveillance on the road, formed on the optical sensor, is scanned in a direction defined in a vertical plane parallel to the direction of travel of the vehicle. The profile of the light distribution characteristic on a segment of road is obtained by this scanning of the elements of the optical sensor.
According to the invention in a second aspect, apparatus for detecting the state of humidity of a road on which a motor vehicle is travelling under conditions of obscurity such that the vehicle lighting system, including at least one driving light, is in use, the apparatus comprising an optical sensor carried in the vehicle, and imaging means for forming on the optical sensor an image of a zone of the road in front of the vehicle, is characterised in that the imaging means is adapted to form an image of at least one segment of the said zone, oriented in the direction of travel of the vehicle, whereby the image concerned is a profile, in the form of a line or curve, characteristic of the light distribution on the road, in that a reference, consisting of the characteristic profile, in the form of a line or curve, for at least one segment of dry road is stored in a memory means of the apparatus, and in that the apparatus further includes a comparison means for comparing the image received by the optical sensor with the said reference.
Preferably, the optical sensor consists of a CCD strip, or a battery of photodiodes, or a CCD camera, or an MOS camera.
Preferably also, photosensitive cells which constitute the elementary points or pixels of the optical sensor are scanned in a vertical direction, of a direction defined in a vertical plane parallel to the direction of travel.
The headlights (or driving lights) of the vehicle may be systematically oriented in the correct direction with respect to the ground, in particular by means of an automatic attitude correcting device. The characteristic curve for the segment which serves for reference purposes may be generated when the road is dry and then stored in a memory.
The apparatus of the invention quantifies the humidity levels on the road.
Preferably, the optical sensor consists of a camera having an angular field (in particular of the order of 60xc2x0), sufficient to render it unnecessary to adjust the orientation of the camera with respect to the ground. In this connection, such an angular field enables variations in inclination of the vehicle with respect to the ground, which are generally less than 10xc2x0, to be absorbed.
However, it is preferable that the optical sensor consists of a camera installed in a said driving light and subjected to the same automatic attitude correction as the driving light, which is systematically oriented in the correct direction with respect to the ground by automatic correction apparatus.
Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of a preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings.